In the field of industrial automation the use of one-finger pneumatic grippers is known, such as those shown in the Applicants' 2015 catalog accessible via the Internet at the following page: http://www.gimatic.com/Gimatic/ProductsFiles/Catalogs/it/of.pdf.
These are so called angular grippers since the only jaw they are provided with, hereinafter termed “clamping finger”, is rotatable between a vertical open position aligned to the gripper body itself, and a horizontal closed position angled or transversal with respect to the gripper body. In practice, in the closed position the clamping finger cantileverly protrudes from the gripper body.
The finger movement is imparted by a pneumatic piston that moves in the gripper body, thus acting as a cylinder, in response to the force applied by, usually filtered, compressed air, that can be lubricated or not. The finger is hinged to the gripper body and is constrained to the piston by means of a sliding pin rotating in a slot of the piston itself.
Usually the one-finger grippers are used in the manufacturing industry in order to temporarily restrain workpieces or during the respective packaging thereof. To this effect they are also named “clamping” grippers.
The one-finger grippers presently available on the market all have more or less the same structure and are assembled as it will be now described.
The gripper body, also acting as cylinder, is a circular tube made of metal—usually aluminum—with open and threaded ends. During the assembling, a first end of the body is closed by screwing a first ring nut provided thereto with a gasket; the first ring nut has a hole for the compressed air passage and is indeed connectable to a supply line. The piston, provided with the respective circumferential gasket to ensure the leak tightness, and a spring functioning for bringing the piston back to its initial position when the compressed air supply is halted, are both inserted into the body. At the second end of the gripper body a second ring nut provided with gasket is then screwed. The second ring nut provides the spring with a stationary abutment surface, in order to allow the spring to be compressed under the action of the piston. The second ring nut has a through hole in which a part of the piston, which will be hereinafter named piston foot, is slidingly accommodated. The clamping finger is hinged to the second ring nut; the hinge axis is skew with respect to the axis of the gripper body and, thus, also to the piston axis. The clamping finger is also constrained to the piston foot by means of a pin parallel to the just described hinge and inserted in a slot obtained in the piston foot.
The functioning is simple. Considering the case in which the gripper is initially in the open position, that is to say with the clamping finger vertical, the compressed air supplied through the first ring nut causes the piston to draw back towards the second ring nut. In this phase, the spring is compressed against the second ring nut and the piston foot biases the clamping finger to rotate until bringing it to the closed position, where it remains for as long as compressed air is supplied to the gripper. At the moment the supplying is halted and the pressure inside the gripper decreases due to the compressed air vent, the spring brings the piston back in the open position.
Presently available gripers have certain limitations.
Firstly, the assembling times are long, due to the ring nuts having to be equipped with the respective gaskets centered with respect to the corresponding ends of the tubular body of the gripper and thus screwed while care being taken of complying with a specific tightening torque.
Secondly, often the ring nuts are the gripper weakness. Grippers have to stand millions of operating cycles; sometimes breaking occur at the ring nuts, for example the thread that keeps them constrained on the gripper body breaks, and the pneumatic tightness is therefore lost.
Last but not least, considering that the grippers are made in different sizes in order to offer a variety of powers, it is required for the manufacturer to provide a number of storehouse ring nuts adequate to allow all the new grippers to be assembled and the used grippers to be repaired.